1. Field of the Invention
The present invention relates to a Hall-effect device driver for use in a magnetic sensor, a magnetic recording medium reader, or the like, for correcting the temperature characteristic of sensitivity of a Hall-effect device to produce a detected signal having a voltage level which is accurately representative of a magnetic flux intensity.
2. Description of the Prior Art
Hall-effect devices detect magnetic fluxes and output a voltage corresponding to the intensity thereof according to the Hall effect. It has heretofore been known that if a Hall-effect device comprises a thin film of GaAs semiconductor, then the voltage of a detected signal produced from its output terminal depending on the magnetic fluxes that are applied tends to vary due to varying characteristics of the GaAs semiconductor. To avoid such detected voltage variations, it has been customary to adjust the drive current supplied to the Hall-effect device when the detected signal is amplified. For the drive current adjustment, a driver circuit for producing a predetermined output voltage with respect to a given magnetic flux intensity is integrally fabricated in combination with the Hall-effect device as an integrated circuit. The drive current for the Hall-effect device is adjusted by the driver circuit to provide an initial current setting at the time the Hall-effect device is manufactured and also to compensate, from time to time, for time-dependent changes in the drive current so that the Hall-effect device can produce a desired output voltage.
FIG. 1 of the accompanying drawings shows a conventional Hall-effect device driver, which is generally designated by the reference numeral 1, for driving a Hall-effect device 2 having an input terminal 2a connected to a constant-voltage regulated power supply Vcc. As shown in FIG. 1, the conventional Hall-effect device driver 1 includes a field-effect transistor (FET) 3 having a source connected to another input terminal 2b of the Hall-effect device 2, and a resistor RY connected between the drain of the FET 3 and ground. The conventional Hall-effect device driver 1 also has an operational amplifier 4 having an output terminal connected to the gate of the FET 3 and an inverting input terminal connected to the drain of the FET 3, for supplying a setting voltage for setting a drive current I, a voltage divider composed of a variable resistor VR and a resistor RX which are connected in series between the constant-voltage regulated power supply Vcc and ground, for applying a variable divided voltage to the noninverting input terminal of the operational amplifier 4, and impedance-matching resistors R1, R2 connected respectively to output terminals 2c, 2d of the Hall-effect device 2.
The conventional Hall-effect device driver 1 further includes an operational amplifier 5 having noninverting and inverting input terminals connected respectively to the resistors R1, R2, and connected across an amplification factor setting resistor R3, for amplifying a detected signal from the Hall-effect device 2 and outputting the amplified signal, and an operational amplifier 6 for applying a preset voltage to the inverting input terminal of the operational amplifier 5 to allow the operational amplifier 5 to amplify the detected signal at a predetermined amplification factor. Furthermore, the conventional Hall-effect device driver 1 has an impedance-matching resistor R4 connected between the output terminal of the operational amplifier 6 and the inverting input terminal of the operational amplifier 5, resistors R5, R6 connected in series between the constant-voltage regulated power supply Vcc and ground, for applying a divided voltage to the inverting input terminal of the operational amplifier 6, and an output terminal 7 for outputting the amplified detected signal from the operational amplifier 5. The Hall-effect device driver 1 and the Hall-effect device 2 are integrally fabricated as an integrated circuit.
The conventional Hall-effect device driver 1 operates as follows:
When a drive current I is supplied from the constant-voltage regulated power supply Vcc to the Hall-effect device 2, the drive current I flows from the input terminal 2a through the Hall-effect device 2 to the input terminal 2b, from which it flows through the source and drain of the FET 3 and the resistor RY. The Hall-effect device 2 is now energized by the supplied drive current I. When magnetic fluxes are applied to the Hall-effect device 2, the Hall-effect device 2 develops a detected signal across the output terminals 2c, 2d. The detected signal is supplied through the resistors R1, R2 to the operational amplifier 5. The operational amplifier 5 amplifies the applied signal at an amplification factor that has been set by the operational amplifier 6, and outputs the amplified signal from the output terminal 7. The drive current I flowing through the Hall-effect device 2 is controlled as follows: the voltage from the constant-voltage regulated power supply Vcc is variably divided by the variable resistor VR and the resistor RX, and the variably divided voltage is applied to the noninverting input terminal of the operational amplifier 4. To the inverting input terminal of the operational amplifier 4, there is applied a voltage from a junction between the FET 3 and the resistor RY, i.e., a voltage drop developed across the Hall-effect device 2 when the drive current I flows therethrough. These voltages applied to the inverting and noninverting input terminals, respectively, of the operational amplifier 4 serve to control the drive current I that flows through the FET 3 into a predetermined constant value.
The drive current I can thus be adjusted to a predetermined constant value to provide an initial current setting at the time the Hall-effect device 2 is manufactured and also to compensate, from time to time, for time-dependent changes in the drive current I.
If the Hall-effect device 2 is made of a GaAs semiconductor, for example, it has a negative temperature characteristic of sensitivity whose curve has a gradient of about -600 ppm (percent per million). Therefore, the voltage level of the detected signal from the output terminals 2c, 2d of the Hall-effect device 2 varies as the ambient temperature varies. When the voltage level of the detected signal varies with temperature, it does not accurately represent the intensity of magnetic fluxes applied to the Hall-effect device 2.